[cssom-view] scrollIntoView({}) dictionary default should match scrollIntoView(true)

[annevk]

Why isn't the IDL as follows:

void scrollIntoView(optional (boolean or ScrollToOptions) arg = false);

Overloading is better to avoid when possible.

And the dictionary having different defaults than false also seems wrong. Passing {} shouldn't have different results from passing undefined or passing nothing at all.

(Via https://bugzilla.mozilla.org/show_bug.cgi?id=1389274.)

[zcorpan]

(Commenting here instead of creating a duplicate issue.)

https://drafts.csswg.org/cssom-view/#extension-to-the-element-interface

annevk suggests the dictionary defaults for scrollIntoView should be the same as omitting the argument and passing in true (which is start-nearest). Currently the dictionary defaults are instead center-center, because in https://www.w3.org/Bugs/Public/show_bug.cgi?id=17152 it was argued to be a common case so there should be a convenient way to get center-center.

Due to legacy we can't change the behavior of omitted argument or passing true, but I think we can still change the dictionary defaults since they are not yet interoperably implemented. I also think having the defaults match between dictionary and boolean makes for a less surprising and easier to learn API.

Test case	EdgeHTML	Blink	Gecko	WebKit
http://software.hixie.ch/utilities/js/live-dom-viewer/saved/5300	top	center	top	top

Current spec:

dictionary ScrollIntoViewOptions : ScrollOptions {
  ScrollLogicalPosition block = "center";
  ScrollLogicalPosition inline = "center";
};

Suggested change:

dictionary ScrollIntoViewOptions : ScrollOptions {
  ScrollLogicalPosition block = "start";
  ScrollLogicalPosition inline = "nearest";
};

Related: #1367

[zcorpan]

cc @saschanaz @bzbarsky @travisleithead @RByers

[bzbarsky]

Having {} match undefined sounds fine to me from first principles; it's certainly least-surprising. I can't speak to whether that trumps the reasoning from https://www.w3.org/Bugs/Public/show_bug.cgi?id=17152

[zcorpan]

This issue is blocked on
#1505
web-platform-tests/wpt#6253

[zcorpan]

OK I've now pushed fixes for this issue in the above-referenced PRs.